1. Field of the Invention
An aspect of the invention relates to an apparatus for supporting the driving of a vehicle such as an automobile or the like, and more particularly, to an apparatus that performs driving support in a vehicle that is equipped with a steering assist mechanism and a right/left braking/driving force distribution mechanism (a direct yaw moment control mechanism, a torque vectoring mechanism or the like) while attempting to coordinate operation control of the mechanisms and a driver's steering with each other.
2. Description of Related Art
In the field of driving control of vehicles such as automobiles and the like, various types of vehicle driving support systems or automatic driving systems for controlling steering mechanisms and acceleration/deceleration mechanisms of the vehicles have been proposed so that drivers can more easily drive the vehicles. For example, in lane keeping assist (LKA) control, with a view to preventing a running vehicle from deviating from a lane, when the vehicle is about to drift from the lane, the driver is warned through a warning buzzer or the like. Besides, when radar cruise control is in operation, the support for the driver's steering is provided such that the vehicle runs along the lane while performing speed control for maintaining the vehicle speed at a set speed without requiring the driver to continue to depress an accelerator pedal. Besides, in the case of intelligent parking assistant (IPA), the support for steering is provided in parallel-parking the vehicle or putting the vehicle into a garage. Furthermore, in U.S. Pat. No. 4,173,292, there is disclosed a configuration that makes it easy for a driver to adjust the steering angle to an optimal steering angle calculated based on information on a surrounding environment, by changing the steering reaction force of a steering wheel in such a manner as to approach the optimal steering angle, in changing lanes. In Japanese Patent Application Publication No. 2010-042741 (JP 2010-042741 A), there is disclosed a driving support method that is configured to determine a target steering angle based on information on a periphery of a vehicle, increase a steering assist force in the same direction as the target steering angle, and reduce the steering assist force in the opposite direction of the target steering angle. Furthermore, in Japanese Patent Application Publication No. 2000-72021 (JP 2000-72021 A), there is disclosed a driving support method that is configured to calculate a lateral acceleration correction amount for causing a vehicle to move toward a target position determined through the use of an image of an onboard camera, detect a magnitude of a steering amount of a driver, perform steering control in such a manner as to achieve the lateral acceleration correction amount after decelerating the vehicle when the steering amount is smaller than a predetermined threshold, and decelerate the vehicle in such a manner as to achieve the lateral acceleration correction amount after performing steering control when the steering amount is larger than the predetermined threshold.
In general terms, the driving support system or automatic driving system as described above is configured to select either control based only on the system, namely, a mechanical input (the mechanical input - a target input that is mechanically set based on information on the surrounding environment and the like) or control based only on the driver's input (the driver's steering input or a target input that is set based thereon) as operation control of the vehicle, or lead the driver's steering to a target state determined based on the mechanical input. For example, in the case of a cruise control system, when the driver carries out the override of operation control by the system such as steering input, accelerator/brake pedal input and the like for the sake of a right/left turn, lane change or emergency avoidance while the system is in operation, the system stops the operation control according to the mechanical input, and the motion of the vehicle is thereafter controlled based on the driver's input. In the case of the IPA as well, the operation thereof is suspended through the override by the driver's manipulation of the steering wheel. Besides, in the case of the configuration that determines a target value such as an optimal steering angle, a target steering angle or the like based on information on the periphery of the vehicle or the like and that makes it easy to steer the vehicle in the direction toward the target value as described in U.S. Pat. No. 4,173,292 or Japanese Patent Application Publication No. 2010-042741 (JP 2010-042741 A), or the configuration in which it is determined which one of steering control and deceleration of the vehicle is first performed in accordance with the magnitude of the steering amount of the driver in realizing the lateral acceleration correction amount that is determined based on information on the periphery of the vehicle and the like as in Japanese Patent Application Publication No. 2000-72021 (JP 2000-72021 A), the intention or characteristics (how to react to situations, the habit and the like) in the driver's own driving are hardly reflected by the motion of the vehicle during the operation of the driving support control. Therefore, this control can be substantially identified as the control based only on the mechanical input.
The control based only on the mechanical input as described above is expected to enable more efficient driving of the vehicle than the control based only on the driver's input. Besides, while the weight saving of the vehicle is expected from the standpoint of environmental issues and demands for low fuel consumption and low electric power costs, there is an apprehension that the stability against crosswind and road surface disturbances may deteriorate in the case of a lightweight vehicle for reasons of a small moment of inertia in the yaw direction, a deterioration in aerodynamic performance and the like. Therefore, the usefulness of driving support control based on the mechanical input is enhanced. However, if the operation state based only on the mechanical input is maintained in the vehicle in which the system for driving support control as described above is in operation (in this case, the vehicle is in a completely automatic driving state), the steering based on the intention or characteristics in the driver's own driving is substantially not reflected by the motion of the vehicle. Therefore, when the motion of the vehicle is different from a motion expected from the driver's steering, the driver can develop a feeling of strangeness for the difference. In regard to this point, when a steering wheel is not mechanically directly coupled to tires in the configuration of the vehicle, the driver finds it difficult to sensuously grasp a state of the running vehicle during the performance of the control based only on the mechanical input. While it is conceivable to adopt a configuration in which an apparent steering reaction force and an apparent pedal reaction force are given to the driver from a result of the motion of the vehicle through the use of, for example, a steer-by-wire system in order to solve the above problem, the driver still finds it difficult to instinctively perceive the state of the vehicle driven by himself or herself as a result of a delay in acquiring information on the surrounding environment and a delay in control computation. On the other hand, in the case of a vehicle in which a steering wheel is mechanically directly coupled to tires, the driver can keenly and directly feel information on a steering angle and information on a reaction torque, via his or her hands that grip the steering wheel. Meanwhile, for example, when a large control input is realized only by a steering mechanism in entering a curve with a view to causing the locus of the vehicle to follow a mechanically set target locus, a large steering angle input and a large steering reaction force are generated, and the driver who grips the steering wheel feels, via the senses of his or her hands, differences between a steering angle and a torque that the driver himself or herself tries to input and the large steering angle input and the large steering reaction force that correspond to the mechanical control input.
From the standpoint of driving comfort, the driver is prevented from developing a feeling of strangeness due to the performance of the control based on the mechanical input in the driving support system or the automatic driving system as described above. Besides, even in the case where efficient driving of the vehicle can be achieved through the control based on the mechanical input, when the driver's steering is not reflected at all by the driving state of the vehicle, the feeling of strangeness developed by the driver for driving will be increased. However, in the case of a configuration in which the driving state of the vehicle as described above is alternatively selected from the operation based only on the mechanical input and the operation based only on the driver's input, when the operation based only on the driver's input is selected to alleviate the feeling of strangeness developed by the driver or achieve the driver's intention or characteristics, the advantageous effect of the control based on the mechanical input is hardly obtained. Besides, in the case of a configuration in which the driver's steering is led such that the driving state of the vehicle follows the operation based on the mechanical input, when the difference in magnitude or changing pattern between the steering angle input or steering force for realizing the motion state of the vehicle (the target value of the motion of the vehicle) that is determined based on the mechanical input and the steering angle or steering force that the driver himself or herself tries to input becomes large as described above, the feeling of strangeness developed by the driver for the fact that the steering angle or steering force that the driver tries to input, namely, the driver's steering is not reflected is increased.